1. Field of the Invention
The present invention relates to an electric insulated wire used to form a coil in a motor, a transformer, or another electric unit and relates to an insulation coating for electric wires used to form an insulation film of the electric insulated wire.
2. Description of Related Art
An enameled wire, which is one type of electric insulated wire, is formed by applying an insulation coating for electric wires to an electric conductor and by baking the coating to form an insulation film. Coils used in electric units are formed by winding enameled wires of this type; these coils are widely put into practical use as magnet wires for large-capacity, large-sized heavy electric units. As recent electric units have been made compact and their performance has been improved, these units are being controlled by high-voltage inverters.
When these enameled wires are used in the manufacturing of electric units such as, for example, motors and transformers, the enameled wires, in general, have been mainly wound continuously in a coiled shape along slots in the cores of the motors. In another main method, the enameled wires have been wound in a coiled shape and then fitted to and inserted into the slots in the cores.
In a method proposed for an enameled wire having a large cross sectional area, that is, a large outer diameter or for an enameled wire having a rectangular electric conductor, a plurality of short, small-diameter coils with a small number of turns are formed by winding enameled wires and the ends of the enameled wires of each small-diameter coil are welded to those of the adjacent coils to form a long coil, instead of forming a long coil with a large number of turns by continuously winding an enameled wire. Coils formed in this way are used in small electric units that need high-density magnetic fluxes, such as, e.g., electric generators in automobiles.
The main exemplary enameled wires used to form coils in electric generators in automobiles are double-coated enameled wires having a polyamide-imide insulation film around a polyester imide insulation film formed around an electric conductor. Other main examples are single-coated enameled wires having a polyamide-imide insulation film formed around an electric conductor. Some other enameled wires are double-coated enameled wires having a polyamide-imide insulation film around a polyimide insulation film formed around an electric conductor to improve both heat resistance and mechanical strength (see, e.g., JP-A Hei 5(1993)-130759).
In control by an inverter, a high surge voltage generated by the inverter may enter a motor and thereby a motor insulation system may be affected. If inadequate insulation is provided between enameled wires wound in a coiled shape, deterioration of the insulation layers is accelerated by corona discharges. When a voltage at which a corona discharge starts to occur on the insulation film is higher than the surge voltage caused by the inverter, no corona discharge occurs, prolonging the life of the insulation film. To increase the voltage at which a corona discharge occurs, the insulation film may be thickened or may have a low dielectric constant. However, a thick insulation film is not preferable since it largely affects the motor design from the viewpoint of the conductor fill factor. In a known example of reducing the dielectric constant, an insulation coating for electric wires obtained by dissolving a phenol compound in aromatic polyester resin is applied to an electric conductor and baked to cover the electric conductor with an insulation film; the resulting enameled wire has a low water absorption ratio and superior dielectric characteristics (see, e.g., JP-A-2003-16846).
It is also proposed that an insulation coating for electric wires produced from fluorine-based polyimide resin is applied to the surface of an electric conductor and baked (see, e.g., JP-A-2002-56720). JP-A-Hei 10(1998)-289622 discloses a polyimide-silica hybrid electric insulated wire obtained by applying a modified polyimide precursor solution to an electric conductor and by baking the precursor, the modified polyimide precursor solution being prepared by dissolving tetraalkoxysilane and pure water in a polyimide precursor solution.
Although a low dielectric constant can be achieved by the methods described above, a toxic fluorine gas is given off during baking at a high temperature. Another problem is that since a float occurs between the electric conductor and the insulation film due to an inadequate adhesion between them, a dielectric breakdown occurs even when a low voltage is applied.
Enameled wires having an insulation film made of polyamide-imide and enameled wires having an insulation film made of polyimide are classified into a high continuous heat resistance class, i.e., a 180° C.-220° C. class, so these enameled wires are widely used as heat-resistant enameled wires. Although polyamide-imide and polyimide are highly heat resistant, they have high dielectric constants and dielectric dissipation factors due to their high polarity.